Mass and decays of Brout-Englert-Higgs scalar with extra generations

The higher bound on the mass of the Brout-Englert-Higgs scalar boson arising from radiative corrections is not stable when the Standard Model is extended to include nondecoupling particles. In particular, additional generations of fermions allow for a heavier scalar. We investigate how the decay branchings of the scalar boson are affected by the opening of new channels. The text was submitted by the authors in English.     

Top quark mass in exophobic Pati-Salam heterotic string model

We analyse the phenomenology of an exemplary exophobic Pati-Salam heterotic string vacuum, in which no exotic fractionally charged states exist in the massless string spectrum. Our model also contains the Higgs representations that are needed to break the gauge symmetry to that of the Standard Model and to generate fermion masses at the electroweak scale. We show that the requirement of a leading mass term for the heavy generation, which is not degenerate with the mass terms of the lighter generations, places an additional strong constraint on the viability of the models. In many models a top quark Yukawa may not exist at all, whereas in others two or more generations may obtain a mass term at leading order. In our exemplary model a mass term at leading order exist only for one family. Additionally, we demonstrate the existence of supersymmetric F- and D-flat directions that give heavy mass to all the colour triplets beyond those of the Standard Model and leave one pair of electroweak Higgs doublets light. Hence, below the Pati-Salam breaking scale, the matter states in our model that are charged under the observable gauge symmetries, consist solely of those of the Minimal Supersymmetric Standard Model.     

Higher-order soft corrections to squark hadro-production

We present new predictions for the total cross section of squark pair-production at Tevatron and LHC through next-to-next-to-leading order within the Minimal Supersymmetric Standard Model. The results are based on the numerically dominant soft corrections. They are exact in all logarithmically enhanced terms near threshold, include the Coulomb corrections at two loops and exact scale dependence. We translate the increased total cross section at next-to-next-to-leading order into improved exclusion limits for squark masses and we investigate the scale dependence as well as the sensitivity on the parton luminosity.     

QCD Effects in Higgs Physics

Higgs boson production at the LHC within the Standard Model and its minimal supersymmetric extension is reviewed. The predictions for decay rates and production cross sections are updated by choosing the present value of the top quark mass and recent parton density sets. Moreover, all relevant higher order corrections, some of which have been obtained only recently, are included in a consistent way.     

Higgs production at the LHC: an update on cross sections and branching ratios

New theoretical and experimental information motivates a re-examination of the Standard Model Higgs production rates at the LHC pp collider. We present calculations of the relevant cross sections and branching ratios, including recently calculated QCD next-to-leading order corrections, new parton distributions fitted to recent HERA structure function data, and new values for electroweak input parameters, in particular for the top quark mass. Cross sections are calculated at two collider energies, √s = 10 TeV and 14 TeV.     

Neutrino masses from gauge symmetries

A very simple extension of the Standard Model to include an Abelian family symmetry is able to describe the hierarchy of quark and lepton masses and their mixing angles together with the unification of gauge couplings. We consider the implications of this model for neutrino masses and mixing angles and show that they are determined up to a discrete ambiguity corresponding to the representation content of the Higgs sector responsible for the Majorana mass matrix.     

Precise prediction for the mass of the lightest Higgs boson in the MSSM

The leading diagrammatic two-loop corrections are incorporated into the prediction for the mass of the lightest Higgs boson, m_h, in the Minimal Supersymmetric Standard Model (MSSM). The results, containing the complete diagrammatic one-loop corrections, the new two-loop result and refinement terms incorporating leading electroweak two-loop and higher-order QCD contributions, are discussed and compared with results obtained by renormalization group calculations. Good agreement is found in the case of vanishing mixing in the scalar quark sector, while sizable deviations occur if squark mixing is taken into account.     

Update of the global electroweak fit and constraints on two-Higgs-doublet models

We present an update of the global fit of the Standard Model electroweak sector to latest experimental results. We include new kinematic top quark and W boson mass measurements from the LHC, a \(\sin \!^2\theta ^{\ell }_{\mathrm{eff}}\) result from the Tevatron, and a new evaluation of the hadronic contribution to \(\alpha (M_Z^2)\). We present tests of the internal consistency of the electroweak Standard Model and updated numerical predictions of key observables. The electroweak data combined with measurements of the Higgs boson coupling strengths and flavour physics observables are used to constrain parameters of two-Higgs-doublet models.     

General scan in flavor parameter space in models with vector quark doublets and an enhancement in the B→X_sγ process

In models with vector-like quark doublets, the mass matrices of up and down type quarks are related.Precise diagonalization of the mass matrices has become an obstacle in numerical studies. In this work we first propose a diagonalization method. As its application, in the Standard Model with one vector-like quark doublet we present the quark mass spectrum and Feynman rules for the calculation of B → X_sγ. We find that i) under the constraints of the CKM matrix measurements, the mass parameters in the bilinear term are constrained to a small value by the small deviation from unitarity; ii) compared with the fourth generation extension of the Standard Model, there is an enhancement to the B → X_sγ process in the contribution of vector-like quarks, resulting in a non-decoupling effect in such models.     

Rephasing-invariant parametrization of the quark mixing matrix

We suggest that within the context of the standard model with three generations, the most convenient way of parametrizing the Kobayashi-Maskawa matrix is through four independent moduli of its matrix elements. For three generations the strength of CP violation is completely determined from the knowledge of a set of independent moduli. We extend this parametrization to the case of four generations and analyze in detail the unitarity constraints on the quark mixing matrix. These unitarity constraints are expressed in terms of independent moduli of the quark mixing matrix and can be a useful tool in obtaining precise experimental tests of unitarity.     

Standard model Higgs-boson branching ratios with uncertainties

We present an update of the branching ratios for Higgs-boson decays in the Standard Model. We list results for all relevant branching ratios together with corresponding uncertainties resulting from input parameters and missing higher-order corrections. As sources of parametric uncertainties we include the masses of the charm, bottom, and top quarks as well as the QCD coupling constant. We compare our results with other predictions in the literature.     

Precise determination of the Z resonance parameters at LEP: “Zedometry”

This final analysis of hadronic and leptonic cross-sections and of leptonic forward-backward asymmetries in collisions with the OPAL detector makes use of the full LEP 1 data sample comprising of integrated luminosity and selected Z decays. An interpretation of the data in terms of contributions from pure Z exchange and from Z interference allows the parameters of the Z resonance to be determined in a model-independent way. Our results are in good agreement with lepton universality and consistent with the vector and axial-vector couplings predicted in the Standard Model. A fit to the complete dataset yields the fundamental Z resonance parameters: GeV, GeV, nb, , and . Transforming these parameters gives a measurement of the ratio between the decay width into invisible particles and the width to a single species of charged lepton, . Attributing the entire invisible width to neutrino decays and assuming the Standard Model couplings for neutrinos, this translates into a measurement of the effective number of light neutrino species, . Interpreting the data within the context of the Standard Model allows the mass of the top quark, GeV, to be determined through its influence on radiative corrections. Alternatively, utilising the direct external measurement of as an additional constraint leads to a measurement of the strong coupling constant and the mass of the Higgs boson: and GeV. Received: 1 December 2000 / Published online: 6 April 2001     

Enhancement of CP-violating phenomena at tree level due to heavy quark propagator singularities

We discuss new interesting CP-violating phenomena in scattering processes resulting from heavy quarks with masses larger than the W-mass (m _q >M _W ). Such a situation is connected with a singularity in the heavy quark propagator which, properly regularized, gives rise to measurable CP-odd effects. The observability of these CP-odd signals in the most representative Kobayashi-Maskawa (KM) models (like fourth generation model, left-right symmetric model and two Higgs doublet model) is investigated. We find that the necessary conditions for measurable CP-asymmetries imply the enlargement of the Standard Model (SM) in the generations of quarks. In addition a significant mixing between the fourth and the other generations is required.     

Measurement of the strange quark forward-backward asymmetry around the Z^{0} peak

A precise measurement of the strange quark forward-backward asymmetry used 3.2M multihadronic events around the Z peak collected by the DELPHI experiment from 1992 to 1995. The ring imaging Cherenkov detectors in the barrel and end-cap regions identify high energy charged kaons which tag the s quark. The s quark asymmetry was measured at different centre-of-mass energies; in particular at the Z pole taking the expected d and u quark asymmetries from the Standard Model. The quark flavour fractions are assumed from the Standard Model and the fragmentation process is modelled by JETSET. From the s quark pole asymmetry the electroweak mixing angle was determined: The parity violating coupling of the s quark to the Z was determined to be: Received: 8 October 1999 / Revised version: 23 February 2000 / Published online: 18 May 2000     

Gluino decays in the complex MSSM: a full one-loop analysis

We evaluate all two-body decay modes of the gluino, in the Minimal Supersymmetric Standard Model with complex parameters (cMSSM). This constitutes an important step in the cascade decays of SUSY particles at the LHC. The evaluation is based on a full one-loop calculation of all two-body decay channels, also including hard QED and QCD radiation. The dependence of the gluino decay to a scalar quark and a quark on the relevant cMSSM parameters is analyzed numerically. We find sizable contributions to the decay widths and branching ratios. They are, roughly of O(±5%)\mathcal{O}({\pm 5\%)}, but can go up to ±10% or higher, where the pure SUSY QCD contributions alone can give an insufficient approximation to the full one-loop result. Therefore the full corrections are important for the correct interpretation of gluino decays at the LHC. The results will be implemented into the Fortran code FeynHiggs.     

The masses of the neutral {\cal CP}-even Higgs bosons in the MSSM: Accurate analysis at the two-loop level

We present detailed results of a diagrammatic calculation of the leading two-loop QCD corrections to the masses of the neutral -even Higgs bosons in the Minimal Supersymmetric Standard Model (MSSM). The two-loop corrections are incorporated into the full diagrammatic one-loop result and supplemented with refinement terms that take into account leading electroweak two-loop and higher-order QCD contributions. The dependence of the results for the Higgs-boson masses on the various MSSM parameters is analyzed in detail, with a particular focus on the part of the parameter space accessible at LEP2 and the upgraded Tevatron. For the mass of the lightest Higgs boson, , a parameter scan has been performed, yielding an upper limit on which depends only on . The results for the Higgs-boson masses are compared with results obtained by renormalization group methods. Good agreement is found in the case of vanishing mixing in the scalar quark sector, while sizable deviations occur if squark mixing is taken into account. Received: 11 January 1999 / Published online: 28 May 1999     

Higgs radiation off top quarks in high-energye + e − colliders

Higgs particles can be radiated off heavy top quarks which will be produced copiously in high energye ⁺ e ^? colliders. This process can be used to measure the Higgs-top quark coupling. We present the cross section for the production of Higgs bosons in the Standard Model. In addition we have studied the production of neutral and charged Higgs particles in association with heavy fermions in the Minimal Supersymmetric Standard Model.     

Could there be a fourth generation of quarks without more leptons?

We investigate the possibility of adding a fourth generation of quarks. We also extend the Standard Model gauge group by adding another SU(N) component. In order to cancel the contributions of the fourth generation of quarks to the gauge anomalies we must add a generation of fermions coupling to the SU(N) group. This model has many features similar to the Standard Model and, for example, includes a natural generalisation of the Standard Model charge quantisation rule. We discuss the phenomenology of this model and, in particular, show that the infrared quasi-fixed point values of the Yukawa coupling constants put upper limits on the new quark masses close to the present experimental lower bounds.     

Supersymmetric correction to top quark pair production near threshold

We studied the leading supersymmetric contribution to top–antitop threshold production using the NRQCD framework. The one-loop matching to the potential and the Wilson coefficient of the leading ³S₁ production current were considered. We point out that the leading correction to the potential is zero due to SU(3)_c gauge invariance. This is true in general for any new physics that enters above the electroweak scale. The shape of the top quark pair production cross section is therefore almost unaffected near threshold, allowing a precise determination of the top quark mass based on the Standard Model calculations. The supersymmetric correction to the Wilson coefficient c₁ of the production current decouples for heavy super particles. Its contribution is smaller than the Standard Model next-to-next-leading-log results.     

Higgs interchange and bound states of superheavy fermions

Hypothetical superheavy fourth-generation fermions with a very small coupling with the rest of the Standard Model can give rise to long enough lived bound states. The production and the detection of these bound states would be experimentally feasible at the LHC. Extending, in the present study, the analysis of other authors, a semirelativistic wave equation is solved using an accurate numerical method to determine the binding energies of these possible superheavy fermion-bound states. The interaction given by the Yukawa potential of the Higgs boson exchange is considered; the corresponding relativistic corrections are calculated by means of a model based on the covariance properties of the Hamiltonian. We study the effects given by the Coulomb force. Moreover, we calculate the contributions given by the Coulombic and confining terms of the strong interaction in the case of superheavy quark bound states. The results of the model are critically analysed.